Sensitization of photographic silver halide emulsions



United States Patent D 6 Claims. oi. 96-107) ABSTRACT OF THE DISCLOSURE Sensitivity of photographic silver iodobromide emulsions is increased by polyglycol esters of spirocyclic erythritol di-phosphoric acid. These esters can be polymeric and can be present either in the emulsion or in the developer bath.

A number of methods have previously been described for increasing the sensitivity of photographic silver halide emulsions. The incorporation of certain colored compounds or dyes in the emulsions increases the optical range of sensitivity, and for this reason such dyes are commonly referred to as optical or spectral sensitizing dyes. It is also well known to increase the sensitivity of photographic emulsions by addition of sulfur compounds capable of reacting with silver salts to form silver sulfide, or by addition of reducing agents (compounds of these types are also naturally present in gelatin), or with salts of gold or other noble metals, or with combinations of two or more of the aforementioned compounds generally known as chemical sensitizers. Such chemical sensitizers are believed to react with the silver halide to form, on the surface of the silver halide, minute amounts of silver sufide or of silver or of other noble metals, and these processes are capable of increasing the sensitivity of emulsions by very large factors. The process of chemical sensitization, however, reaches a definite 'limit beyond which further addition of sensitizer, or of further digestion with the sensitizer present, merely increases the fog of the photographic emulsion leaving the sensitivity constant or even decreasing it.

Other chemical sensitize-rs are higher polyethylenglycols having a molecular weight of at least 1500. Lower polyethylen lycols have no or only very small effect on the sensitivity of silver halide emulsions. While the higher polyethylenglycols have a considerable or remarkable sensitization effect, the utility of such compounds is limited by the fact that they cause marked increases in the fog levels of silver halide emulsions. It is known that the fog level increase can not be controlled by the addition of antifoggants to the emulsion.

It is among the objects of the present invention to increase the general sensitivity or speed of silver halide emulsion layers. A further object is to .provide photographic silver halide emulsions and emulsion layers having an increased general sensitivity without concomitant increases in fog levels. Other objects will be apparent from the following description of the invention.

By the term general sensitivity it is intended to refer to the sensitivity of photographic emulsions through the whole spectrum. By the term spectral sensitivity it is intended to refer to the sensitivity of the photographic emulsion to light of a certain range of the spectrum.

The foregoing objects are attained by the use of polyethylene glycol esters of spirocyclic pentacrythritol-bis- (phosphoric acid partial esters. The aforesaid products increase the general sensitivity of silver bromo iodide "ice emulsions containing between 1 and 10 mol percent of silver iodide.

The effect of the polyethylenoxid derivatives of the pentaerythritol esters of phosphoric acid are obtained if the development of the silver halide emulsion layers is performed in effective contact with those sensitizers. The scnsitizers of the present invention are particularly superior to known higher polyethylenglycols since they do not cause any fogging of the photographic silver halide emulsions. Thus, the silver halide emulsions containing the sensitizers of the present invention have improved stability on keeping.

The esters of the invention of spirocyclic phosphoric acid esters of the formula:

wherein X represents chlorine or bromine, with a polyethyleneglycol of the general formula:

wherein n is 3 to 100, preferably 3-50, the said glycols preferably being used in quantities of about l-2 mols per mol of the halides of Formula I.

The condensation products which are particularly preferred are those of the spirocyclic phosphoric acid esters of Formula I with the following polyethylene glycols:

Hexaethylene glycol Nonaethyleue glycol Octadecaethylene glycol, and

Polyglycols which contain about 30-100 ethylene oxide groups.

The quantity of the resulting phosphoric acid esters to be added to the emulsion depends on the ratio of chloride to bromide in the emulsion, and 10 to 100 mg. per mol of silver halide may advantageously be used. The emulsions can in addition be chemically sensitised with salts of noble metals, such as gold salts, with sulfur compounds or with reducing agents. Moreover the emulsions can be optically sensitised with cyanine and merocyanine dyes.

The preparation of some of the esters of the present invention is given below:

Compound I (a) 136 g. of pentaerythritol are introduced into 500 cc. of phosphorus oxychloride and heated in a steam bath with stirring. The pentaerythritol dissolves during 30 to 45 minutes, hydrochloric acid thus being formed, and the spirocyclic pentaerythritol-di-(phosphoric acid monochloride) of the formula (Ia) O-CHz CHz-O O \II -Cl O-Ga \CH2O is deposited in the form of colourless crystals, melting point 243-245 C. The crystals may be isolated by suction filtration after some of the phosphorus oxychloride has been removed by evaporation in vacuo, and the crystals may then be washed with methylene chloride on the suction filter.

(b) 30 g. of the ester chloride (Ia) described above are added in portions with stirring to a mixture of 37.2 g. octaethyleneglycol and 31.2 g. of pyridine, the temperature gradually rising to 35 C. After 4 hours the mixture is poured onto ice, adjusted to pH 3 with 2 N H treatedi with saturated sodium chloride solution, and extracted with methylene chloride. After drying with potash, the methylene chloride is evaporated off completely. 25 g. of a light brown highly viscous oil is obtained, which is diluted with water to a 10% solution for photographic use. The following theoretical formula is derived from the proportions of the starting materials and the molecular weight of 900 determined by osmosis:

(O-CH CH -OH radicals.

Compound II 19.4 g. of tetraethylene glycol 31.2 g. of pyridine and 30 g. of ester chloride (Ia) are reacted as described above and worked up. 27 g. of a viscous light brown oil are obtained. The colour may be removed from the 10% aqueous solution by means of active charcoal and bleaching earth. The oily liquid presumably consists mainly of a compound of the following theoretical formula:

Varying the reaction time and proportion of components has the same effect as disclosed in connection with the prescription for the preparation of Compound I.

Compound III The compound is prepared by a similar method to that used for compound (Ib) by reacting 28.2 g. of hexaethyleneglycol and 31.2 g. of pyridine with 30 g. of ester chloride (Ia). Yield 17 g. A possible theoretical formula of the end product would be:

41.4 g. of nonaethyleneglycol, 31.2 g. of pyridine and 30 g. of ester chloride (Ia) produce 20 g. of a brown syrupy polycondensate which is diluted with water to a 10% solution and purified with active charcoal and bleaching earth. The following theoretical formula may be used to represent the highly viscous polycondensate:

75.7 g. of polyethyleneglycol (with 34 oxyethylene units), 15.6 g. of pyridine and g. of ester chloride (Ia) are stirred for 2 hours at room temperature and for 1 hour at 40 C. with exclusion of moisture. The viscous reaction mixture is then poured onto ice, slightly acidified with 2 N-sulphuric acid and extracted with methylene chloride. After evaporating off the methylene chloride, 36 g. of a semi-solid mass are obtained. The following theoretical formula may be assumed for the reaction product:

6 g. of Compound Ia and 40 g. of a polyethyleneglycol consisting of ethylene ether units are heated in a vacuum of about 15 mm. of mercury to a temperature of to C. while stirring, hydrogen chloride escaping in large quantities. After the reaction is completed the reaction mixture is made neutral with a methanolic sodium alcoholate solution. The alcohol is removed again in vacuo. 42 g. of a colorless, waxy product are obtained.

The preparation of the spirocyclic carbon compounds used for comparson in the examples will now be described.

Preparation: from pentaerythritol, pyruvic acid and ethanol (Ber. 61 (1928), page 1856).

Compound VHI 16.6 g. of Compound VII and 18.5 g. of octa-ethylene glycol are heated for 2 hours at C. in the presence of 0.2 g. of sodium methylate, the ethyl alcohol which is liberated distilling off. The reaction product is diluted with water to a 10% solution and neutralised with 1 N hydrochloric acid. Osmotic determination of the molecular weight indicated that the value of X was 2.

The reaction products can also be obtained by heating the components to about 100-180 C. in vacuo and continuously removing the hydrogen chloride as a gas which is set free during the reaction. The compounds obtained by the above reaction are generally water-soluble.

The preparation of photographic silver halide emulsions includes 3 separate steps:

(1) Emulsification and physical ripening which is also called Ostwald ripening; (2) the freeing of the emulsion of excess water soluble salts, usually by washing with water and drying, and (3) the after ripening which is also called chemical ripening to obtain increased emulsion speed or general sensitivity.

The sensitizers of the present invention can be added to the emulsion before, during or after the chemical ripening or they can be added immediately prior to the casting.

The particular quantity of the sensitizers of the invention used in a given emulsion can vary, depending upon the effects desired, silver content of the emulsion, the silver halide composition etc. Generally they are added in amounts of 10 to 200 mg. per mol of silver halide. The compounds are also effective when they are added to the developer solution, in which case they are preferably used in quantities of between approximately 30 and 600 mg. per liter of the aqueous developer bath.

The sensitizers can be dissolved in water or a solvent miscible with water or a mixture of water and watermiscible solvents, and added in this form to the emulsion. The solvent is not critical and should be selected so that it should have no harmful effect to the photographic properties of the silver halide emulsion.

The optimum amount for any sensitizer of the present invention can be determined for any particular emulsion by running a series of comparison tests in which the quantity of the sensitizer is varied over a given range. Exposure of the emulsion containing the sensitizer in a manner well known and measuring of the sensitivity in conventional apparatuses will reveal most advantageous concentrations. Such technique is well understood by those skilled in the art.

The photographic emulsion in which the sensitizers according to the invention are used, can be chemically sensitized by any of the known procedures. They can be sensitized, for example, with sulfur compounds as re ferred to, e.g., in the book The Theory of the Photographic Process by Mees (1954), pages 149-161.

The emulsions can also be chemically sensitized with salts of noble metal such as gold, ruthenium, rhodium, palladium, iridium and platinum, used in amounts below that which produce any substantial fog. Representative compounds are ammonium chloropalladate, potassium chloroplatinate, potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, aun'c trichloride and the like. The emulsions can be sensitized with reducing agents, stannous salts or polyamines and the like. The emulsions can also be optically sensitized with cyanine, merooyanine or rhodacyanine dyes.

The emulsions can be stabilized with any of the known stabilizers, such as mercury compounds, triazoles for example mercapto phenyl triazoles, or azaindenes as described, for example, by Birr in Z.wiss.phot. vol. 47 (1952) pages 2 to 28. The silver halide emulsion layers may contain any suitable film-forming and waterpermeable colloid as binding agent, such as gelatin but the gelatin can be replaced partially with products like alginic acid and derivatives thereof such as salts, preferably with alkali metals, esters with lower aliphatic alcohols or amides. The gelatin can also be partially replaced with polyvinyl alcohol, polyvinyl pyrrolidone, starch, carboxymethyl cellulose and the like.

The following examples will serve to illustrate more fully the manner of sensitizing photographic silver halide emulsions with the sensitizers of the present invention.

Example 1 An ordinary silver bromo iodide gelatin emulsion containing per kg. of casting solution 0.3 mol of silver halide with an iodine content of 6 mol percent and which further contains the usual additives such as wetting agents, optical sensitizers, stabilizers and hardeners, was divided into 4 portions.

Sample A is used as control sample. To the other samples were added the following sensitizers:

Sample B: 160 mg. of nonaethylene glycol,

Sample C: 160 mg. of a condensation product of 1 mol of compound IV,

Sample D: 200 mg. of compound VI,

Sample E: 160 mg. of compound I.

The various portions were then coated on a transparency support such as cellulose acetate and dried.

The samples were exposed in a sensitometer behind a test wedge and developed for 10 minutes in a developer of the following composition at 18 C.:

G. Anhydrous sodium sulfite 70 Borax (N3-2B40710H2O 7 Hydroquinone 3.5 Monomethyl-p-aminophenol 3.5 Sodium citrate 7 Potassium bromide 0.4

With water to 1 liter.

The above results show the sensitizing effect of the compounds of the present invention. Short-chain polyethylenglycols (Sample B) have no eflect.

Example 2 An ordinary silver bromo iodide gelatin emulsion containing per kg. of casting solution 0.3 mol of silver halide with an iodine content of 6 mol percent and which further contains the usual additives such as wetting agents, optical sensitizers, stabilizers and hardeners, was cast onto a suitable transparent support of a polyester of terephthalic acid and ethylenglycol. The dried ligh*-sensitive ma terial is divided into 2 strips. Both samples were exposed in a sensitometer behind a test wedge and developed for 10 minutes at 18 C. One sample (Sample F) was developed in the developer composition of Example 1 and the other sample (Sample G) in a developer composition corresponding to that of Example 1, but with an additional content of 400 mg. of Compound V per liter of developer solution.

1. In the process for the production of photographic images by exposing silver halide emulsion layers in which the silver halide consists of a mixture of silver bromide with up to 10 mol percent of silver iodide and developing the exposed layer, the improvement which comprises performing the development with the emulsion in operative contact with a sensitizing amount of a condensation product of a spirocyclic pentaerythritol-di-(phosphoric acid monohalide) with a polyethylenglycol having between 3 and ethylenglycol units.

2. The combination of claim 1 wherein the polyethyleneglycol component of the condensation product has between 3 and 50 ethylenglycol units.

3. In the process for the production of photographic images by exposing silver halide emulsion layers in which the silver halide consists of a mixture of silver bromide with up to 10 mol percent of silver iodide and developing the exposed layer, the improvement which comprises performing the development with the emulsion in operative contact with a sensitizing amount of a compound having the formula:

wherein R represents a member of the group consisting of hydrogen and a radical of the formula n represents an integer between 3 and 100 and X represents an integer between 1 and 100.

4. A light-sensitive silver halide emulsion, the silver halide of which consists of a mixture of silver bromide and up to 10 mol percent silver iodide, the emulsion containing a sensitizing amount of a condensation product of a spirocyclic pentaerythritol-di-(phosphoric acid monohalide) with a polyethylene glycol having between 3 and 100 ethylenglycol units.

5. A light-sensitive photographic material as defined in claim 4, wherein the polyethylenglycol has between 3 and 50 ethylenglycol units.

6. A light-sensitive silver halide emulsion the silver halide of which consists of a mixture of silver bromide and up to 10 mol percent silver iodide, the emulsion containing 21 sensitizing amount of a compound having the References Cited formula: UNITED STATES PATENTS I" 1? p 3 fi -K3 3,169,863 2/1965 Grabhofer et a1 96107 Io1 o 1(0oH2oH1)n J OH 5 FOREIGN PATENTS 0-0 CHT'O 635,167 1/1964 Belgium. wherein R represents a member of the group consxstlng of hydrogen and a radical of the formula NORMAN G. TORCHIN, Primary Examiner.

{CH -CH -O% H 10 I. H. RAUBITSCHEK, R. FICHTER, n represents an integer between 3 and 100 and X repre- Asslsmm Emmmem' sents an integer between 1 and 100. 

